Malignant glioma is a nearly uniformly lethal cancer of the central nervous system that is largely refractory to conventional modalities of therapy. One promising new approach is the use of a genetically engineered, conditionally replicating herpes simplex virus (HSV) following conventional therapy to kill remaining tumor cells. G207 is a recombinant HSV lacking genes essential for replication in normal brain cells. G207 has been shown to be efficacious in the treatment of both human and rodent tumors in murine models. Safety has been established following intracranial inoculation in the Aotus monkey (Aotus nancymai). We hypothesize that G207, a genetically-engineered HSV-1, will be safe and potentially useful for the treatment of patients with malignant gliomas who have evidence of progressive disease on MRI despite treatment with conventional surgery and/or biopsy and external beam radiation. The mode of administration will be stereotactic inoculation of virus into an enhancing region of the tumor.